CoreDNS实现自定义域名解析

参考：https://support.huaweicloud.com/usermanual-cce/cce_01_0361.html

一、修改CoreDNS Hosts配置

修改CoreDNS配置文件，将自定义域名添加到hosts中。

例如将www.example.com指向192.168.1.1，通过CoreDNS解析www.example.com时，会返回192.168.1.1。

须知：
此处配置不能遗漏fallthrough字段，fallthrough表示当在hosts找不到要解析的域名时，会将解析任务传递给CoreDNS的下一个插件。如果不写fallthrough的话，任务就此结束，不会继续解析，会导致集群内部域名解析失败的情况。
hosts的详细配置请参见https://coredns.io/plugins/hosts/。

[root@k8s01-zongshuai dev]# kubectl edit configmap coredns -n kube-system
# Please edit the object below. Lines beginning with a '#' will be ignored,
# and an empty file will abort the edit. If an error occurs while saving this file will be
# reopened with the relevant failures.
#
apiVersion: v1
data:
  Corefile: |
    .:53 {
        errors
        health {
           lameduck 5s
        }
        ready
        kubernetes cluster.local in-addr.arpa ip6.arpa {
           pods insecure
           fallthrough in-addr.arpa ip6.arpa
           ttl 30
        }
        hosts {
           172.16.43.178   bx-cdh01
           172.16.43.187   bx-cdh02
           172.16.43.189   bx-cdh03
           172.16.43.192   bx-cdh04
           172.16.43.193   bx-cdh05
           172.16.43.177   cdh01
           172.16.43.199   cdh02
           172.16.43.184   cdh03
           172.16.43.182   cdh04
           172.16.43.159   cdh05
           fallthrough
        }
        prometheus :9153
        forward . /etc/resolv.conf
        cache 30
        loop
        reload
        loadbalance
    }
kind: ConfigMap
metadata:
  creationTimestamp: "2021-11-30T07:36:28Z"
  managedFields:
  - apiVersion: v1
    fieldsType: FieldsV1
    fieldsV1:
      f:data: {}
    manager: kubeadm
    operation: Update
    time: "2021-11-30T07:36:28Z"
  - apiVersion: v1
    fieldsType: FieldsV1
    fieldsV1:
      f:data:
        f:Corefile: {}
    manager: Mozilla
    operation: Update
    time: "2022-09-07T07:11:00Z"
  name: coredns
  namespace: kube-system
  resourceVersion: "112726088"
  selfLink: /api/v1/namespaces/kube-system/configmaps/coredns
  uid: 218630e6-42f7-4e64-a0cf-cf97ac6bc6d6

在CoreDNS中修改hosts后，就不用单独在每个Pod中配置hosts了，带来了一定的方便性。　　

二、添加CoreDNS Rewrite配置指向域名到集群内服务

使用 CoreDNS 的 Rewrite 插件，将指定域名解析到某个 Service 的域名，相当于给Service取了个别名。

修改CoreDNS配置文件，将example.com指向default命名空间下的example服务。

$ kubectl edit configmap coredns -n kube-system
apiVersion: v1
data:
  Corefile: |-
    .:5353 {
        bind {$POD_IP}
        cache 30
        errors
        health {$POD_IP}:8080
        kubernetes cluster.local in-addr.arpa ip6.arpa {
          pods insecure
          fallthrough in-addr.arpa ip6.arpa
        }
        rewrite name example.com example.default.svc.cluster.local
        loadbalance round_robin
        prometheus {$POD_IP}:9153
        forward . /etc/resolv.conf
        reload
    }
kind: ConfigMap
metadata:
  creationTimestamp: "2021-08-23T13:27:28Z"
  labels:
    app: coredns
    k8s-app: coredns
    kubernetes.io/cluster-service: "true"
    kubernetes.io/name: CoreDNS
    release: cceaddon-coredns
  name: coredns
  namespace: kube-system
  resourceVersion: "460"
  selfLink: /api/v1/namespaces/kube-system/configmaps/coredns
  uid: be64aaad-1629-441f-8a40-a3efc0db9fa9　

三、使用CoreDNS级联自建DNS

修改CoreDNS配置文件，将forward后面的/etc/resolv.conf，改成外部DNS的地址

$ kubectl edit configmap coredns -n kube-system
apiVersion: v1
data:
  Corefile: |-
    .:5353 {
        bind {$POD_IP}
        cache 30
        errors
        health {$POD_IP}:8080
        kubernetes cluster.local in-addr.arpa ip6.arpa {
          pods insecure
          fallthrough in-addr.arpa ip6.arpa
        }
        loadbalance round_robin
        prometheus {$POD_IP}:9153
        forward . 192.168.1.1
        reload
    }
kind: ConfigMap
metadata:
  creationTimestamp: "2021-08-23T13:27:28Z"
  labels:
    app: coredns
    k8s-app: coredns
    kubernetes.io/cluster-service: "true"
    kubernetes.io/name: CoreDNS
    release: cceaddon-coredns
  name: coredns
  namespace: kube-system
  resourceVersion: "460"
  selfLink: /api/v1/namespaces/kube-system/configmaps/coredns
  uid: be64aaad-1629-441f-8a40-a3efc0db9fa9